"Once Upon a Time" fans aren't the only ones dying to see the big wicked vs. evil showdown in Sunday's (April 6) episode, "It's Not Easy Being Green." The Wicked Witch herself -- Rebecca Mader -- has been counting down the days until she could see how the big episode turned out.

Mader has been having a blast playing the big bad villain,
the Wicked Witch/Zelena, on the ABC drama so far. "It is so much fun. I just can't believe it," Mader says. "I've enjoyed playing this character so much because she is just so wicked. Sometimes I'll be doing a scene and I'll just be like, 'Gah. I'm awful! Oh god!' She is just
so awful. And that's why she's so fun to play, because she's so incredibly different from who I am. It's always so fun to play someone really opposite to you."

When it came to portraying such an iconic character, Mader approached the role with a specific goal: To make Zelena a different kind of villain than Lana Parrilla's Regina. "I didn't want to just come in and just be like Regina. I needed to differentiate her," Mader says. "I use the word 'wicked' all the time and it's such a popular word in the U.K., and it can sometimes mean a little bit naughty and cheeky. So I've been using that English 'wicked' element with this character to make her more cheeky at the center of it all."

Mader had so much fun playing the Wicked Witch that it started to take on a life of its own. "It's funny because I didn't realize how giggly I'd been with it until I'm now watching it," she says with a laugh. "I'll do something dreadful and then laugh, and it's making me laugh because I didn't realize that I was even doing that. I'm just like, 'Oh god, I'm horrible!'"

Mader promises a "monumental" fight scene in Sunday's episode. "The big showdown was just a massive scene to shoot. It was a lot of work," she says. "We were outside in Storybrooke until about six o'clock in the morning -- it was a big, big, big, big night shoot. And it was physically grueling. We had all these stunt people there, it was freezing cold, but it was really fun. We put so much hard work into it, that I'm just really excited to watch it on Sunday night and see how it all turned out."

While Mader can't reveal who initiates the showdown, she does reveal that Zelena tells Regina about their familial connection again. "I'm telling her again in this episode because I know she doesn't remember it from when I told her a year ago," Mader says. "It's all a part of my plan and my endgame and my control. It's moving forward with my plan."

Another reason Mader is so excited to see the episode is the flashbacks to Oz. "I think it's going to be fun for the audience to go to Oz," she says. "People have been waiting for that since the Wicked Witch came on the show, so on Sunday night we'll finally get to see Oz. We'll see when I discover that I have this half-sister, and I want to leave Oz and go to Fairy Tale Land. It's this great backstory about who I was and where I was and why I've come to Fairy Tale Land and why I've come to Storybrooke. Plus we'll meet Dorothy and Glinda the Good Witch in the next couple of weeks."

Mader hopes that seeing Zelena's backstory will allow the audience to empathize with her. "I love that they've told my character's backstory. It's always fun seeing the villain's backstory," Mader says. "Like when we saw Rumpelstiltskin when he was nice, before he became the Dark One. You get this empathy for him, and there's nothing better than a villain that you can feel for, rooting for someone that's broken and damaged. I think people can relate to that because nobody is perfect.

"They've written the villains so well on this show with Regina and Rumpel, when you get to see why they've become the way that they are it's so interesting. So that's what I'm most excited for on Sunday night, for people to see why I've become green and why I've become wicked."

So what does Mader enjoy playing more, the Oz/Fairy Tale Land flashbacks or the Storybrooke present? "I've enjoyed both. But I have to say, I love the Wicked Witch stuff," Mader says with a laugh. "I do really enjoy being green. I really enjoy flying on my broomstick. But I do enjoy being the white version of me too, being in Storybrooke, especially when they don't know who I am and I'm walking around pretending to be this wonderful midwife and the audience is screaming, 'Run! No!' I think that's really, really fun having the audience in on who I am."